This document discusses the MOS capacitor structure. It begins by defining the key components of a MOS capacitor as a semiconductor substrate, insulating oxide layer, and conducting metal layer. The document then examines the band diagram of an n-type silicon MOS capacitor at zero bias. It explains how applying a positive or negative bias to the metal layer can cause the band diagram to deviate, resulting in accumulation, depletion, or inversion at the silicon surface. Key relationships between the metal and silicon surface potentials are also defined for a p-type silicon MOS capacitor under varying bias conditions.

1. MOS Capacitor
Arpan Deyasi
Dept of ECE, RCCIIT, Kolkata, India
11/3/2020 1Arpan Deyasi, RCCIIT, India

2. 11/3/2020 Arpan Deyasi, RCCIIT, India 2
Are MOS & MOSFET are same?
No
Let’s make the journey to the world of MOS

3. 11/3/2020 Arpan Deyasi, RCCIIT, India 3
Substrate
Insulator
material
Conducting
Layer
Consider a semiconducting substrate
Formation of MIS structure
Grow insulating material on top of
the substrate
Make a metal layer deposition on the
insulating layer
Apply bias on the two contact layers

4. 11/3/2020 Arpan Deyasi, RCCIIT, India 4
MIS MOS
Semiconductor
substrate
Oxide
Conducting
Layer
Insulating layer will be the oxide layer
Convention:
Conducting layer: Metal (Al)
Insulating layer: Oxide (SiO2)
Substrate layer: Semiconductor (Si)

5. 11/3/2020 Arpan Deyasi, RCCIIT, India 5
To understand its electronic properties,
we need to understand it’s band diagram
Since it is a heterostructure,
we have to consider vacuum level as the reference level
Q: What is heterostructure?

6. 11/3/2020 6
Need to brush up with Band Structure?
Work Function (Φs)
Electron Affinity (χs)
Arpan Deyasi, RCCIIT, India

7. 11/3/2020 Arpan Deyasi, RCCIIT, India 7
Band Diagram of MOS [Al-SiO2-Si(n)]
EFm
χ =
4.25
eV
ECI
χ = 0.9 eV
EVI
Eg =
8.76
eV
ECS
EVS
Eg =
1.12
eV
χ =
4.05
eV
EFS
ΦF
E0
(Al) (SiO2) (Si)

8. 11/3/2020 Arpan Deyasi, RCCIIT, India 8
This is the band diagram at unbiased condition
Q. If bias is applied,
how the band diagram will be deviated?

9. 11/3/2020 Arpan Deyasi, RCCIIT, India 9
Application of bias in MOS Structure
1. Positive bias at metal side
2. Negative bias at metal side

10. 11/3/2020 Arpan Deyasi, RCCIIT, India 10
Property of semiconductor will play a vital role
[i] Al-SiO2-Si(n)
[ii] Al-SiO2-Si(p)
1. Positive bias at metal side
1. Positive bias at metal side
2. Negative bias at metal side
2. Negative bias at metal side

11. 11/3/2020 Arpan Deyasi, RCCIIT, India 11
Al-SiO2-Si(p) under different biasing conditions
Al SiO2 Si
VM < 0
Al SiO2 Si
ΦAl
ΦSi
Ec
EV
EFP
Ei
χSiO2
χSi
Ec
EV
EFP
Ei
Accumulation
ΨF
ΨS

12. 11/3/2020 Arpan Deyasi, RCCIIT, India 12
Al-SiO2-Si(p) under different biasing conditions
Al SiO2 Si
ΦAl
ΦSi
Ec
EV
EFP
Ei
χSiO2
χSi
Al SiO2 Si
VM>0 & small
Ec
EV
EFP
Ei
Depletion
ΨF
ΨS

13. 11/3/2020 Arpan Deyasi, RCCIIT, India 13
Al-SiO2-Si(p) under different biasing conditions
Ec
EV
EFP
Ei
Al SiO2 Si
VM>0 & small
Depletion
VM>0 & large
Al SiO2 Si
1
2
point of
inversion
3
Inversion
ΨF
ΨS

14. 11/3/2020 Arpan Deyasi, RCCIIT, India 14
Relation between ΨS & ΨF for Al-SiO2-Si(p)
Accumulation: ΨS < ΨF & ΨS <0
Depletion: ΨS < ΨF
Point of Inversion: ΨS = ΨF
Inversion: ΨS > ΨF

15. 11/3/2020 Arpan Deyasi, RCCIIT, India 15
Bending of band for Al-SiO2-Si(p)
Accumulation: upward bending
Depletion: Downward bending
(EFp does not cross EFi)
Point of Inversion: Downward bending
(Efp touches Efi)
Inversion: Downward bending
(EFp crosses EFi)